Scientists know that temperature determines sex in certain reptiles—alligators, lizards, turtles, and possibly dinosaurs. In many turtles, warm temperatures during incubation, create females; cold temperatures, males. This is one hypothesis that explains dinosaur extinction: a meteor hit, cooling the Earth, producing an all-male population. The reason ancient reptiles like turtles and alligators survived, the hypothesis goes, is that the water regulated drastic temperature changes during incubation.
This subject has fascinated Northland College Assistant Professor of Biology Kayla Bieser, Ph.D., since she went abroad as an undergraduate in Costa Rica researching leatherback sea turtles. She pursued graduate degrees studying red-eared slider turtles in Indiana and Alabama. Her most recent research, completed with Thane Wibbels, Ph.D., her mentor at the University of Alabama at Birmingham, will be published by December 2014 in the primary research journal Sexual Development.
This study represents the most comprehensive simultaneous evaluation of the chronology of how sex-determining genes express themselves during embryonic development and, maybe more interestingly, at least to Bieser, the study also looks at the impacts of estrogen.
She’s interested in the genetics behind sex determination. Scientists know that temperature determines sex in turtles. In the species she studies, a swing in only a couple degrees Celsius is the difference between becoming male or female. Researchers even understand that this happens during the middle-third of the incubation period, which can be approximately 20-35 days long depending upon the incubation temperature.
But no one understands why temperature influences sex—whether it’s a hormonal reaction, a genetic one, or a combination of factors. Bieser is seeking to answer that question.
Bieser and Wibbels followed five different genes and what was going on in the exact same turtle. To date, scientists have looked at a number of turtles and pooled the data, but Bieser is the first to follow individual turtles. She wanted to know when and how they “express” themselves. For an example, Bieser describes expression as the physical manifestation of those genes such as blue or brown eye color.
She looked at turtle eggs incubated at male and female temperatures and looked at what the genes were doing while sex is being determined. “Which genes ‘turn on’ and when, could be an indication of what is triggering sex,” Bieser said.
Bieser also applied estrogen to eggs at a male-producing temperature. The purpose she said is to help determine the triggers for sex determination and how hormones, such as estrogen, can override the temperature signal.
In other words, would temperature or estrogen win out in deciding sex? The answer: in short, neither. What she found—and this is new information—is that when estrogen is applied to eggs incubating at a male temperature, gonads, male or female, do not develop. Or, if they do, they barely develop.
Why? “ We don’t know yet,” Bieser said.
Scientists have been doing this experiment for some time but never reported these results. She suspects the reason is because scientists did not dissect the gonadal area specifically and that they dissected the general area but may have not analyzed the gonads to the same detailed level. In fact, this was a sticking point for one of the reviewers of this study—so Bieser provided photos of her findings.
Bieser, who joined Northland College in fall 2014, has continued and expanded her research with northern turtles—painted turtles and snapping turtles. She and biology student Caitlin Ginnery worked on two research studies this past summer.
Ginnery took Bieser’s Methods of Molecular Biology and “fell in love with genetics.” In one semester, she went from being pre-med to wanting to focus on genetics and is now looking at graduate programs in genetic research, pediatric genetics, and genetics counseling.
Last summer, Bieser hired Ginnery to work with her in the lab. The two researched epigenetics, which look “above the genome,” to see how genes are expressed without changing the letters—“an exact, abstract, and invisible science,” Bieser said.
Bieser and Ginnery were also able to duplicate the results from Bieser’s initial estrogen study.
Bieser has no intention of slowing down. She’s wildly interested in environmental estrogens and how they impact people and wildlife. She is working with other faculty at Northland College to set up studies to look at turtles near a sewer outlet, for example, versus turtles living in more pristine conditions.
“This research provides a critical understanding of how temperature acts on and above the genes in species where temperature determines sex—this is particularly critical in light of global climate change,” Bieser said.
According to Bieser, temperature-dependent sex determination species may be unable to evolve rapidly enough to offset the increases in temperature, which may ultimately result in their extinction.
“It’s critical that we understand the genetic mechanisms on which temperature acts and incorporate this knowledge into management plans for the conservation of these vulnerable species.”